1. Field of the Invention
The present invention relates to a laser machining apparatus for machining a workpiece using a laser beam and an assist gas.
2. Related Art
In general, when a laser machining apparatus cuts a metal sheet workpiece, an assist gas is ejected from a nozzle of a machining head onto a portion to be machined while a laser beam is being emitted. Here, a material of the portion of the workpiece that is to be machined is melted or evaporated by the laser beam and is removed by the assist gas. Therefore, the workpiece to be machined is supported using a multi-pin type support means, such that the laser beam does not strike members for supporting the workpiece.
When the machining is performed in this way, a phenomenon in which the portion of the workpiece to be machined is sequentially pressed away from the nozzle by the assist gas from the nozzle and then is returned by elasticity tends to occur. When this phenomenon occurs, the portion to be machined vibrates and a distance to the nozzle is changed. Accordingly, machining conditions are not maintained constant, thereby making accurate machining difficult.
This problem becomes significant, in particular, when machining is started from an edge of the workpiece. That is, at a portion of the workpiece edge in which the machining is started, the distance between the nozzle and the portion to be machined is greatly changed due to a weak supporting force provided by a multi-pin type support means, and owing to the assist gas being abruptly ejected. Therefore, instability in machining conditions becomes significant. In addition, there is a problem in that, for example, the nozzle and the workpiece interfere with each other because the portion that is to be machined is significantly displaced.
In order to effectively avoid such a problem caused by the ejection of the assist gas, machining is conventionally performed at a low speed. In addition, it is also known to perform machining by providing a support means for locally supporting a workpiece by surrounding a machining position below the machining head, and then by sequentially and locally supporting respective portions of the workpiece that are to be machined.
For example, in a laser cut machining device disclosed in Patent Document 1 (JP-A-2003-103388), a support means for supporting an object to be machined is composed of a holder, which surrounds a nozzle of a machining head and of which a distance to the lower end is identical with a focal length of a laser beam, and a support plate, which brings the object to be machined into close contact with the holder by urging the object upward. When machining, the object to be machined is supported by an XY stage and is moved between the machining head and the holder and between the machining head and the support plate. At this time, the holder and the support plate locally support a portion of the object to be machined that is currently positioned between the holder and the support plate.
However, when machining a steel strip produced by sequentially uncoiling a coil material, it is required to move a machining head with respect to a workpiece. In this case, in order to apply the local support means for locally supporting the workpiece as disclosed in Patent Document 1 in the machining of the coil material and while supporting width ends of the workpiece, it is required to provide a mechanism that moves the support means together with the machining head in a wide range that covers the width ends of the workpiece.
Since this mechanism increases the cost of the apparatus due to the increased scale, the local support means is not suitable for the case in which the coil material is machined as a workpiece. In addition, when a multi-pin type support means, which supports entire portions of the workpiece from below, is used, the foregoing local support means cannot be employed since the local support means interferes with pins of the multi-pin type support means.